OUT

V

2

PD

F

I

R

u u

RF 54.9k

Ÿ

CF 2.7pF

VOUT

-

+

OPA2156

+12V

RF 54.9k

Ÿ

CF 2.7pF

-

+

OPA2156

-12V

+12V

+

-

21

OPA2156

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8.2 Typical Application

The combination of low input bias, high slew rate and a rail-to-rail input and output enable the OPA2156 to serve

as an accurate differential photodiode transimpedance amplifier. This application example shows the design of

such a system.

Figure 46. OPA2156 Configured as a Differential Photodiode Transimpedance Amplifier

8.2.1 Design Requirements

The design requirements for this design are:

•

Photodiode current: 0 µA to 90 µA

•

Output voltage: –5 V to 5 V

•

Supply voltage: ±12 V

•

Filter cutoff frequency: 1 MHz

8.2.2 Detailed Design Procedure

In this example the OPA2156 serves as a transimpedance amplifier for a differential photodiode. The differential

configuration allows for a wider output range (0 to 10-V differential) compared to a single-ended configuration (0

V to 5 V). This output can be connected to a differential successive approximation register (SAR) analog-to-

digital converter (ADC). The basic equation for a differential transimpedance amplifier output voltage is shown in

Equation 3.

(3)

Equation 3 can be rearranged to calculate the value of the feedback resistors as shown in Equation 4.